1. Field of the Invention
The present invention relates to boundary layer control devices for use with VTOL or STOL aircraft, and more particularly to boundary layer control devices mounted on the wings of VTOL and STOL aircraft for minimizing the separation of layers of air flowing over airfoil surfaces of the wing.
2. Description of the Prior Art
One of the more critical aerodynamic considerations in the design of VTOL and STOL aircraft is the behavior of the flow of air over the lift- generating surfaces of these aircraft. FIGS. 1 and 2 of the accompanying drawings present two views of a typical VTOL/STOL aircraft 10 showing rotors 20, 20' mounted on the wings 30 at a location near the wingtips. The position of the rotors 20,20' shown in the Figures is one associated with a VTOL (i.e., for "take-off", "landing" or "hovering") mode of operation. When the aircraft is operating in a "cruise" mode, each respective rotor and engine housing 25 will have been pivoted through an angle of about 90.degree. to a position where the axis of rotation of the rotors is substantially horizontal and parallel to the longitudinal axis of the aircraft fuselage.
Early attempts to minimize the resistance of air flowing over aircraft lift surfaces, in particular wing surfaces in STOL aircraft, resulted in structure such as that illustrated in FIG. 3 of applicant's drawings. As seen, the conventional primary flap 45 is combined with a secondary trim flap 48. The primary flap is pivotally connected to the wing 30 for rotation between a first position in which the longitudinal axis of the wing and primary flap are substantially coincident, and a second position in which the longitudinal axes of the wing and the primary flap are disposed at a maximum angle of about 45.degree.. The secondary trim flap 48 is pivotally connected to the primary flap, and appears to enjoy a similar range of movement relative to the primary flap, so that when the primary and secondary flaps are in their second positions, the secondary flap and the wing make an angle of about 90.degree..
NASA Report No. TM 84401, entitled "AIRLOADS ON BLUFF BODIES, WITH APPLICATION TO THE ROTOR-INDUCED DOWNLOADS ON TILT-ROTOR AIRCRAFT", authored by W. J. McCroskey et al., discusses the problems associated with a slightly different form of the same problem, i.e., the impingement of the wake of a lifting rotor on a horizontal surface, such as a wing, fuselage or control surface. The conclusion of the report is that this airflow denigrates the lifting capabilities of the aircraft in hover and low-speed flight. Such vertical drag phenomenon, also known as "download", is particularly important for tilt-rotor configurations since both the downwash velocities of the rotors and the affected wing area are larger than for conventional helicopters. The NASA Report presents evidence which indicates that minimum download does not occur when the wing flaps of the aircraft ar fully deflected so as to present minimum wing surface area to the rotor downwash, but rather when the flaps are deflected only approximately 60.degree.. This behavior, it is surmised, is caused by flow separation on the upper surface of the flaps, and such flow separation might be minimized by maintaining the wing flaps at the optimum angle of about 60.degree. (see, for example, the disposition of flap 26 relative to wing 30 in FIGS. 2 and 4 of applicant's drawings).
Through various testing, it has also been determined that download can be minimized through the use of flap and "rotating cylinder" boundary layer control devices. These mechanisms, which contribute to diminishing flow separation of air passing over the trailing and/or leading surfaces of the airfoil, comprise a rapidly rotating cylindrical or tubular element disposed at the edge of the airfoil (generally at the trailing edge) married via appropriate linkage or couplings with the pivoting flap.
Typically the rotating cylinder boundary layer control device s coupled with or incorporates the drive shaft interconnecting the rotors supported on each of the wings of the aircraft. A power source carried by the aircraft causes the drive shaft, and hence the rotating cylinder, to rotate at high speeds. Typically, the rotating cylinder is driven in rotation at speeds of from about 6,000 RPM to about 9,700 RPM, and tests conducted by NASA have demonstrated that a cylinder speed of about 7,500 RPM will prevent separation of the airflow from the airfoil surfaces.
One of the earliest examples of a rotating cylinder boundary layer control device which combines a "pivoting flap" is disclosed in U.S. Pat. No. 3,179,354 to Alvarez-Calderon, and is shown schematically in FIG. 5 of applicant's drawings. In the illustrated configuration, a rotating cylinder is provided which spans the length of each of the wings and extends along their trailing edges. The apparatus is coupled to, and supported by, the fixed portion of each of the aircraft wings (shown in cross-section as element 56) and includes a pivotally mounted "flap" 57 on each of the wings. A shaft 55 defines a pivot axis for the "flap", and a rotating cylinder 58 extends across the entire span of each of the wings and rotates about the pivot axis. The "flap" consists of a pivoting wing portion which carries one or more engine-driven rotors 60. The shaft which defines the pivot axis transmits power from a remote power source (e.g., an internally housed engine) to a propeller drive element 59 (shown in phantom in FIG. 5) by means of a gear box 50. The propeller drive element in turn causes rotation of the rotors which, in the Alvarez-Calderon aircraft, are pusher propellers. These pusher propellers are pivotally movable from a location below the wing in the "VTOL" mode of operation and behind the wing in the "cruise" mode of operation. The flap and propeller shaft are designed to pivot approximately 90.degree. from the "cruise" position to the "VTOL" position.
The boundary layer control device of Alvarez-Calderon appears to have been the first meaningful solution to the nagging problem of controlling, if not minimizing, rotor downloading in VTOL aircraft. Nevertheless, this solution was only of limited utility in a configuration where the rotor is above the wing n the "VTOL" mode. The propulsion mechanism used by the patentee was that of a pusher propeller, not a tractor propeller, so that the source of fluid from which the airflow over the wing and flap was derived encompassed all fluid in the ambient. Moreover, the location of the propulsion mechanism relative to the aircraft's fixed wings was rearwardly or below in the Alvarez-Calderon aircraft depending on the mode of operation, rather than forwardly or above as in the more conventional configurations known today.
Therefore, it appears that the patentees never addressed the problem of rotor downwash caused by direct impingement of rotor-driven fluid on the wing or flap of a VTOL/STOL aircraft. While the Alvarez-Calderon invention was a significant step forward in improving the performance of VTOL aircraft, it is clear that the patentee neither understood, nor recognized, the problem of rotor download on the wings or flaps of VTOL/STOL aircraft.
The tilt rotor aircraft configuration taught by the Alvarez-Calderon patent, in which the VTOL rotors are of the push propeller type supported on a flap pivotally mounted to and depending beneath the wing, is fundamentally different from the more conventional tilt rotor aircraft as taught by the present invention which has its VTOL rotor(s) disposed above the wing.
In the configuration of Alvarez-Calderon (FIG. 6), the cross flow (.e., the air flow shown by lines A passing across the top of the wing or flap 56) is of relatively low velocity and the flow past the bottom of the wing is induced by the propeller located below the wing. At the trailing edge of the wing, the cross flow is assisted by the rotating cylinder 58, while the trailing edge "flap" has only been turned 90 degrees from the otherwise horizontally directed "cruise" position.
In contrast, in the configuration of the present invention (FIG. 7), the rotor is above the wing 30, and the cross flow A is of high velocity. In this case, it is important to turn the flow around the trailing edge with the rotating cylinder 38 so the negative pressure will be reduced on the underside of the wing. To do this, the rotating cylinder is exposed to the cross flow so that its surface velocity is 2 to 4 times that of the cross flow and the trailing edge flap is disposed so a not to impede the flow around the edge of the wing. Presumably, a rotating cylinder on the leading edge would also improve the cross flow of air over the wing, especially in the VTOL mode o operation.